(2) In order to make the imagepermanent, it is necessary toremove the unreduced silverbromide from the surface ofthe developed film. This operationis called fixing of image.Fixing is done by dipping thedeveloped film or plate in sodiumthiosulphate (hypo) solution.The hypo solution dissolvesthe unreduced silverbromide by forming a complex.Thus, sodium thiosulphateacts as a complexing agent.